The semiconductor light emitting diode is a p-n junction specially fabricated to emit light under an applied voltage. In conventional inorganic semiconductor devices a p-n junction consists of a single crystal semiconductor formed such that part of the crystal is doped with positively charged carriers called p-type and the other part is doped with negatively charged carriers called n-type. It is a basic feature of all such junctions that their chemical composition and, hence, doping profile are static, or fixed in position in the host crystal. During operation in a device charge carriers are injected into, or removed from, the junction through electrical contacts external to the junction region. Abrupt junctions, in which the transition between the n- and p-type regions is relatively narrow, are typically fabricated by alloying a solid impurity (for example, a metal) with the semiconductor, or by one of a number of epitaxial growth techniques on crystalline semiconductor substrates; graded junctions, in which the transition region is relatively broader, are produced by diffusion or ion implantation of impurities into the host semiconductor. These technologically demanding manufacturing processes make it both difficult and expensive to fabricate large area displays. Moreover, such devices are inherently brittle and lack the mechanical and processing advantages generally associated with organic, and especially polymeric materials. For these reasons, there has been considerable interest for many years in the development of suitable organic materials for use as the active (light-emitting) components of light emitting diodes.
More recently, a number of workers have disclosed electroluminescent devices using organic materials as the active light emitting layer in sandwich architecture devices. For example, S. A. Van Slyke and C. W. Tang in U.S. Pat. No. 4,539,507 disclosed a device consisting of a bilayer of two vacuum sublimed films of small organic molecules sandwiched between two contacts, while R. H. Friend et al. in U.S. Pat. No. 5,247,190 disclosed a device consisting of a thin dense polymer film comprising at least one conjugated polymer sandwiched between two contacts. Because these are electric field driven devices, the active electroluminescent layer must be very thin (about 1000 angstroms thick or less) and uniform. In these devices excess charge carriers are injected through the contacts into the light emitting semiconductor layer by processes well known in the study of metal-semiconductor interfaces see, e.g., M. A. Lampert and P. Mark, Current Injection in Solids, Academic Press, N.Y., 1970!. Dissimilar metals were used for the contacts to facilitate the injection of electrons at one contact and of holes at the other. As a result the current-voltage characteristic curves of these devices show a pronounced asymmetry with respect to the polarity of the applied voltage, like that typical of the response of diodes. Hence, the rectification ratio of such devices is high, typically greater than 10.sup.3, and light is emitted for only one polarity of the applied voltage. Among other drawbacks, the devices disclosed by S. A. Van Slyke and C. W. Tang and by Friend et al. suffer from the need to use metals of relatively low work function to inject sufficient numbers of electrons into the active layers to produce efficient light output at low drive voltages. Because such metals are readily oxidized, they are a source of device degradation in ambient conditions and require passivating packaging.
Electrochemistry provides a convenient means of reversibly doping a number of semiconductors with n- and p-type carriers. This carrier injection mechanism is physically distinct from that in the sandwich architecture electroluminescent devices disclosed by S. A. Van Slyke and C. W. Tang and by Friend et al. In particular, in the case of electrochemical doping the charge carriers generated are compensated by counter-ions from the electrolyte. However, the mobility of these carriers is often too low for practical use. This is believed to be due to the fact that in a semiconductor in contact with an electrolyte, subsequent to an electrochemical oxidation or reduction reaction, the charge carrying species generated are typically ionically bound to a counter-ion from the electrolyte. The electrochemical generation of the charge carriers therefore necessarily also involves incorporation of the compensating counter-ions within the seniconductor. The often dense morphology of many semiconductors inhibits the diffusion of the counter-ions, leading to slow doping and undoping kinetics.
Semiconducting polymers offer particular advantages as electronic materials. These materials exhibit the electrical and optical properties of semiconductors in combination with the processing advantages and mechanical properties of polymers. Inorganic crystalline devices, in contrast with many polymer materials and objects, are mechanically brittle. Semiconducting polymers can often be doped by chemical means with relative ease, and the dopant species can often diffuse into the anisotropic polymer structure at room temperature. Alternatively, the doping can often be carried out electrochemically as a redox reaction, and the doping level controlled by the applied electrochemical potential with respect to a counter-electrode. Although useful as electrodes in battery applications, electrochromic devices, and the like, such electrochemically doped materials have not been considered suitable for semiconductor device applications since the dopant species are mobile at room temperature. As a result, any doping profile (such as that needed for forming a p-n junction) is necessarily transient.
The electrochemiluminescent cell, a device for generating light using the reversible oxidation-reduction reactions of organic or metallo-organic species in an electrochemical cell, is disclosed by A. S. Bard et al. in U.S. Pat. No. 3,900,418. As in the present invention, electrochemiluminescent devices produce light by electron transfer reactions between electrogenerated species. The devices disclosed by Bard et al. rely on an organic solvent containing the electrolyte to transport the oxidized or reduced light emitting molecules themselves, rather than the charge carriers, between the electrodes. The oxidized and reduced species react with each other to form the original organic or metallo-organic species in an electronically excited state which may subsequently decay radiatively. N. Levantis and M. S. Wrighton in U.S. Pat. No. 5,189,549 disclose electrochemiluminescent displays in which the electrochemiluminescent substance is dissolved in a solid electrolyte. Again, N. Levantis and M. S. Wrighton disclose that after generation of the oxidized and reduced species, said species diffuse away from their generation sources (i.e. the electrodes) and eventually meet somewhere between the electrodes. Alternatively, other workers have found that an electrochemiluminescent material can be fixed on one of the electrodes in an electrochemical cell and cyclically reduced and oxidized by an alternating potential. A direct current potential can be used only if the cell contains an additional species which serves to interact with the luminescing material in such a way as either to oxidize it at the same potential at which it is electrochemically reduced or to reduce it at the same potential at which it is electrochemically oxidized see, e.g., M. M. Richter et al., to be published in Chem. Phys. Lett. 226, 115 (1994)!. A key disadvantage of many of these electrochemiluminescent devices is the large volume of organic solvent relative to the quantity of the electrochemiluminescent material, said volume of organic solvent being a source of reactants whose electrochemical side reaction products act to quench the recombination radiation. The use of the solvent is also a disadvantage from the point of view of the fabrication and packaging of such devices. In solid electrolytes, such as polymer electrolytes, the diffusion rate of the oxidized and reduced species are substantially lower, which compromises device performance.
In U.S. Pat. No. 5,682,043, which is incorporated herein by reference, there is disclosed an electrochemical light emitting device. That device includes a composite material in contact with two electrodes. The composite material is an admixture of ionic species and an `immobile` semiconductor. The semiconductor is capable of supporting both p- and n-type carriers and having a doping profile which can be dynamically changed in a controlled fashion through reversible electrochemical oxidation and reduction. Devices having this structure may be used to generate electrochemically induced p-n junctions, thereby providing a new means of exploiting the light emitting properties of such junctions under an applied voltage.